Industrial robots are automatically guided manipulation machines, equipped with three or more freely-programmable axes, which are equipped with appropriate tools for the automatic manipulation of objects. They can be used in mobile or stationary applications. These robots are designed for use in industrial environments (e.g. automotive manufacturing) and generally comprise a manipulator (robot arm), a control device and drive means for moving the manipulator. In many cases, both the drive and the manipulator are monitored by means of sensors. The monitoring contributes to the control and regulation of the drive. The drive can, for example, be an electric drive, which comprises power electronics and an electric motor.
In certain situations, e.g. if a person enters a protected area surrounding the industrial robot, or if pre-determined limits for motor or axis parameters are exceeded, it is necessary to reliably and quickly stop the movement of the industrial robot and thus to dependably stop the axis/axes of the robot. The objective of the dependable stopping is to prevent movements which represent hazards, so as to not endanger machinery, workpieces and, in particular, people.
The standard EN 60204-1 differentiates between three categories of dependable stopping of robots:    category 0: Uncontrolled stopping of the axis through immediate (<200 ms) shutdown of the power supply to the drive.    category 1: Controlled stopping of the axis through interruption of the power supply to the drive, after the axis has been brought to a standstill (=time-delayed shutdown of the power supply).    category 2: Controlled stopping of the axis without interruption of the power supply to the drive.
In many applications, such as, for example, very quickly moved or gravity-loaded axes, a shutdown of the drive energy does not lead to the dependable stopping of the axis, since the axis can coast to a stop or sag as a result of gravity. Additional mechanical brakes are therefore used, conventionally disk brakes, which act, for example, directly on the motor shaft and which are maintained in the released state by means of a current-supplied electromagnet in failure-free operation of the robot. If the operating power is shut down—in the case of a category 0 stop, for instance—the brake is thus automatically applied. A common feature of all mechanical brakes is a certain activation delay. For current disk brakes, this is in a range, for example, of between 100 to 200 ms, before the brake pads come into contact with the brake disk once the braking signal has been sent.
Because the shutdown of the operating power almost immediately, i.e. within a few milliseconds of the command (i.e. after the signal to interrupt the power supply has been sent), leads to a shutdown of the motor, the drive coasts in an uncontrolled manner until the mechanical brake takes effect. Moreover, braking using the mechanical brake is disadvantageous, since high mechanical loads act on the components to be braked.
In order that these disadvantages can be avoided, a short-circuit braking of the motor, which is intended to reduce the coasting of the axis to a stop, is known from WO 2009/074 396 A1, for example. WO 2009/074 396 A1 corresponds to U.S. Pat. No. 8,736,219 B2 to Hofmann et al., which was published in the English language on May 27, 2014 and is hereby incorporated by reference in its entirety herein. To achieve a reduction in the coasting of the axis to a stop, the shut-down of the operating power short-circuits the motor phases of the electric motor, and a braking torque is thus generated. Because short-circuit braking relies, in a known manner, on the induction of current in the motor phases of the motor, the generated braking torque is, however, dependent on the rotational speed of the motor at the time of the shutdown of the operating power. In particular for axes moved slowly or for vertical gravity-loaded axes, the problem of sagging thus essentially remains unchanged.
The objective of the present invention is therefore to provide a method and a system for the dependable stopping of the axis/axes, which eliminates the aforementioned disadvantages and thus permits a quick stopping of the axis, in order to minimize or to avoid the coasting to a stop and the sagging of the axis in particular. This objective is achieved with the method for stopping of axes of an industrial robot according to Claim 1 and with the system for controlling an industrial robot according to Claim 6.